Integrated circuits are formed from a wafer or substrate over which are formed patterned microelectronics layers. In the process of fabricating the integrated circuits, it is common to employ patterned photoresist layers as mask layers for forming those patterned layers from previously deposited blanket layers. After forming one of the patterned layers on the substrate, the corresponding photoresist layer may be removed from the substrate in a stripper chamber or asher before continuing to the next process.
Since photoresist stripping is used frequently in semiconductor manufacturing foundries, strippers or stripper chambers are designed to have very short process times, i.e., high throughput, to reduce the overall substrate manufacturing cost. As such, the performance of a downstream plasma stripper chamber is often determined by its strip rate, which is defined as the rate of photoresist removal per unit time. The strip rate determines how long a substrate is exposed to plasma. As the plasma in the stripper chamber may damage various circuits on the substrate, it is preferred to minimize the substrate's exposure time to ionized gas by increasing the strip rate. Hereinafter, the terms ionized gas and plasma are used interchangeably.
FIG. 1 shows a schematic diagram of a downstream chamber 100 of a stripper chamber. As depicted, a substrate 104 having a photoresist layer to be stripped may be held by a wafer heater chuck 106. Process gas may be energized by a plasma source into plasma 108, enter into the chamber 100 through an opening 110 in the chamber wall 102, and thence passes through the holes in one or more baffles or diffusers 112. The baffles 112 may disperse the gas to improve the gas flow uniformity at the substrate surface. In general, each baffle may contain a large number of holes 122. FIG. 2 shows a top plan view of one of the baffles 112 in FIG. 1. The strip uniformity and the strip rate may be highly dependent upon the baffle configuration. The size and location of the holes 122 in a baffle may be determined to enhance the uniformity of gas flow at the substrate surface. For instance, as depicted in FIG. 2, the sizes of the holes 122 may increase with increasing distance from the center of the baffles 112 because the center of the baffles 112 may receive more gas flow than the edge. In another design to disperse the gas, a showerhead may be used. However, the number and size of holes in the showerhead are such that they typically create a back pressure. The creation of the back pressure may slow down the gas flow above the showerhead and reduce the fluid dynamic efficiency.
The strip rate of the chamber 100 may be adversely affected by several factors. For instance, when the gas or plasma 108 flows through the opening 110, it expands to fill the larger space within the chamber. This expansion may reduce the gas temperature. As the strip rate may increase as the chamber temperature and/or substrate temperature increases, the strip rate may decrease due to the gas expansion. Furthermore, as the gas passes through the baffles 112, it transfers a portion of its heat energy to the baffles 112 and thereby the strip rate is reduced for the same reasons. In addition, recirculation regions 120 may be formed within the chamber. The flow residence time in the recirculation regions 120 may be large enough for a portion of gas radials or ionic species to recombine into neutral species. The recombination process may generate exothermic reaction energy that can be transferred to the chamber wall 102 and baffles 112. Also, the neutral species, which may pass through the holes 122 with the plasma, may not contribute to the removal of the photoresist layer.
Multiple heaters 116 (as shown in FIG. 1) may be installed within the chamber wall 102 to heat up the chamber 100. In general, the heaters of a conventional stripper chamber are used to maintain the chamber temperature at a level to merely prevent wall deposition and, as a consequence, the chamber temperature is kept relatively low. However, if the stripper chamber were to be heated for the purpose of increasing the gas temperature and thereby enhancing the strip rate, it would require an extensive heating and a complicated cooling mechanism installed on the chamber wall and/or thermal insulators 118 to meet the Semiconductor Industry Equipment Safety Requirements, or shortly, SEMI S2. SEMI S2 requires the outside wall temperature be below 60° C. for the safety of human operators. The material for the insulators 118 may be chosen to be clean-room compatible, i.e., it should not shed particles. For example, it would not be acceptable to simply wrap the chamber body with common industrial fiber-glass thermal insulation sheets or blankets. Those requirements typically result in choosing more costly insulation material and/or complex cooling mechanism, adding to the overall complexity of chamber design and manufacturing cost. As such, there is a need for a new stripper chamber that can provide high temperature gas to enhance the strip rate and meet the safety requirements in a cost effective manner.